1. Technical Field
This invention relates generally to digital video signal compression, and more particularly to a method of digital video signal energy compaction method that determines rate distortion performance during the initial motion compensation step, thereby achieving a high signal compression ratio and a low transmission rate.
2. Discussion
Present video applications require large amounts of data to be transmitted at high bit rates and with a minimal amount of signal distortion. For example, the uncompressed data bit rates for digital video, such as VCR-grade video (SIF), broadcast television video (CCIR-601) and high definition television (HDTV) are 4 Mbps, 15 Mbps, and 16Mbps, respectively. In an uncompressed state, these data bit rates are too high to allow such video signals to be transmitted and processed in a commercially feasible manner. Therefore, in order to process such video signals in a practical manner, such video signals must be compressed prior to being transmitted.
In response to the proliferation of video based applications and products, an industry wide need for creation of a standard video signal compression syntax arose. A group under the International Standards Organization (ISO), known informally as the Moving Pictures Experts Group (MPEG), was formed to define standards for digital video and audio compression. MPEG has created a standardized syntax by defining the content of a compressed video signal bit stream and the method of decompressing the bit stream subsequent to its transmission. The methods of compression, however, have not been defined, thus allowing individual manufacturers to develop various methods of actually compressing the data bit stream within the defined standards.
MPEG has to date defined two syntaxes widely used in the digital videos industry. A syntax known as MPEG-1 was defined to be applicable to a wide range of bit rates and sample rates. Particularly, MPEG-1 is suitable for use in CD/ROM applications and other non-interlaced video applications having transmission rates of about 1.5 Mb/s. A second syntax known as MPEG-2 was defined for representation of broadcast video, and other video signal applications having coded bit rates of between 4 and 9 Mb/s. MPEG-2 syntax is also applicable to applications such as HDTV and other applications requiring efficient coding of interlaced video.
While the above discussed MPEG-1 and MPEG-2 syntaxes exhibit adequate performance characteristics, the ongoing evolution of digital video dictates the need for further advancement in the art, as the present MPEG video syntax definitions do have associated limitations. For example, temporal redundancy, a phenomenon which enhances video compression by minimizing data bit rate transmission for non-changing video pixels, is an efficient method of maximizing video signal compression. Present MPEG-1 and 2 data compression-based methods utilize temporal compression.
However, the MPEG-1 and 2-based temporal compression is based on a frame by frame judgement basis so that the methods do not take full advantage of temporal compression. In particular, commercially standard MPEG-1 and MPEG-2 syntaxes only partially utilize temporal redundancy. In addition, present MPEG syntax requires numerous optimization variables (such as predictive frame, bi-linear frame and intraframe variables) to be calculated, transmitted and then decoded by MPEG signal decompressors. The use of these numerous variables adds both computational time and complexity to the data compression. Also, while many current MPEG syntaxes exhibit an associated bit rate compression of as high as 30:1, increasingly complex and data-intensive video applications require higher compression rates for real-time processing. Although data compression methods claiming compression ratios as high as 200:1 do exist, such methods subsample an array of pixels in a video frame sequence (i.e., throw away every other pixel) and utilize other shortcuts to achieve high compression.
Further, in the above-discussed MPEG-1 and MPEG-2 syntaxes, a data compression optimization operation known as Block Matching Algorithm (BMA) is the most computationally intensive operation in the MPEG video compression process. The BMA operation, although it is exhaustive, requires an inordinate amount of processing power. For example, LSI Logic of Milpitas, Calif. presently manufactures a ASIC chip, commercially sold under the name AMEP, which is capable of performing computations at a rate equivalent to 500 Intel Pentium 166 MHz processors. Two of these chips are required just to perform the BMA step, while typically only three additional chips are required to perform the remaining data compression calculations. As a result, the MPEG-2 committee has recently called for new alternative methods to improve upon the existing Block Matching Algorithm (BMA), both to lessen the complexity of the data compression operation and to lower the associated data transmission rate.
Data compression optimization methods which minimize data transmission rate do exist. For example, pending U.S. patent application entitled "Method and System of Three Dimensional Compression of Digital Video Systems", Ser. No. 08/621,855 by Niesen, now U.S. Pat. No. 5,933,143 assigned to TRW, Inc., which is incorporated by reference, discloses video data compression optimization methods performed after video compression transform operations by optimizing video compression based on transform coefficient transmission order. The methods are realized in the final steps of the process after motion compensation and the transform coefficients have been calculated.
While compression methods such as those identified above represent a significant advancement in data compression, there is still room for advancement in the art. In particular, there is a need for a data compression maximization method that is realized in the initial steps of the data compression process to minimize rate distortion as early in the process as possible to thereby optimize the data compression achieved in subsequent steps.